# Trimethylenemethane

Trimethylenemethane (often abbreviated TMM) is a chemical compound with formula C $_4$H $_6$. It is a neutral free molecule with two unsatisfied valence bonds, and is therefore a highly reactive free radical. Formally, it can be viewed as an isobutylene molecule C $_4$H $_8$ with two hydrogen atoms removed from the terminal methyl groups.

## Structure

The electronic structure of trimethylenemethane was discussed in 1948. It is a neutral four-carbon molecule containing four pi molecular orbitals. When trapped in a solid matrix at about 90 K (−183 °C), the six hydrogen atoms of the molecule are equivalent. Thus, it can be described either as zwitterion, or as the simplest conjugated hydrocarbon that cannot be given a Kekulé structure. It can be described as the superposition of three states:
It has a triplet ground state ($^3$A$_2$′/$^3$B$_2$), and is therefore a diradical in the stricter sense of the term. Calculations predict a planar molecule with three-fold rotational symmetry, with approximate bond lengths 1.40 Å (C–C) and 1.08 Å (C–H). The H–C–H angle in each methylene is about 121°.
Of the three singlet excited states, the first one, 1$^1$A$_1$ (1.17 eV above ground), is a closed shell diradical with flat geometry and fully degenerate threefold (D$_{3h}$) symmetry. The second one, 1$^1$B$_2$ (also at 1.17 eV), is an open-shell radical with a D$_{3h}$-symmetric equilibrium between three equal geometries; each has a longer C–C bond (1.48 Å) and two shorter ones (1.38 Å), and is flat and bilaterally symmetric except that the longer methylene is twisted 79° out of the plane (C$_2$ symmetry). The third singlet state, 2$^1$A$_1$/$^1$A$_1$′ (3.88 eV), is also a D$_{3h}$-symmetric equilibrium of three geometries; each is planar with one shorter C–C bond and two longer ones (C$_{2ν}$ symmetry).
The next higher energy states are degenerate triplets, 1$^3$A$_1$ and 2$^3$B$_2$ (4.61 eV), with one excited electron; and a quintet state, $^5$B$_2$ (7.17 eV), with the p orbitals occupied by single electrons and D$_{3h}$ symmetry.

## Preparation

Trimethylenemethane was first obtained from photolysis of the diazo compound 4-methylene-Δ$_1$-pyrazoline with expulsion of nitrogen, in a frozen dilute glassy solution at −196 °C (77 K).
It was also obtained from photolysis of 3-methylenecyclobutanone, both in cold solution and in the form of a single crystal, with expulsion of carbon monoxide. In both cases, trimethylenemethane was detected by electron spin resonance spectroscopy.
Trimethylenemethane has been obtained also by treating potassium with 2-iodomethyl-3-iodopropene and isobutylene diiodide (IH $_2$C)$_2$C=CH $_2$ in the gas phase. However the product quickly dimerizes to yield 1,4-dimethylenecyclohexane, and also 2-methylpropene by abstracting two hydrogen atoms from other molecules (hydrocarbon or potassium hydride).

## Organometallic chemistry

A number of organometallic complexes have been prepared, starting with Fe(C $_4$H $_6$)(CO)$_3$, which was obtained by the ring-opening of methylenecyclopropane with diiron nonacarbonyl (Fe $_2$(CO)$_9$). The same complex was prepared by the salt metathesis reaction of disodium tetracarbonylferrate (Na $_2$Fe(CO)$_4$) with 1,1-bis(chloromethyl)ethylene (H$_2$C=C(CH$_2$Cl)$_2$). Related reactions give M(TMM)(CO)$_4$ (M = Cr, Mo). The reaction leading to (TMM)Mo(CO)$_4$ also gives Mo(C $_8$H $_{12}$)(CO)$_3$ containing a dimerized TMM ligand.
TMM complexes have been examine for their potential in organic synthesis, specifically in the trimethylenemethane cycloaddition reaction with only modest success. One example is a palladium-catalyzed [3+2]cycloaddition of trimethylenemethane.
* Structure of Ru(trimethylenemethane)(CO)$_3$, viewed down C$_3$ axis.
* Structure of Ru(trimethylenemethane)(CO)$_3$, viewed orthogonal to C$_3$ axis.